Blood clot filtering

ABSTRACT

A blood clot filter including an anchoring portion including a generally cylindrical self-expanding body formed from resilient material, the generally cylindrical body having proximal and distal ends and defining an axial direction and having a structure of variable size diameter expandable from a low-profile compressed condition to a larger profile expanded condition, wherein the resilient material urges the generally cylindrical body to radially expand and to thereby apply anchoring radial force against the inner wall surface of the blood vessel; and a generally conical filtering portion axially aligned with the generally cylindrical body having an open proximal end coupled to the distal end of the anchoring portion and having an apical distal end. Also disclosed is a blood clot filter having one or more hooks fixedly coupled to the anchoring portion and formed from compliant material having an original shape that bends under stress yet returns to its original shape when unstressed. The hooks respectively tend to project from the anchoring portion at an acute angle with respect to the axial direction for engagement with a vessel wall. Also, the hooks are deflectable toward the anchoring portion for achieving a low-profile.

FIELD OF THE INVENTION

This invention relates to blood clot filtering.

BACKGROUND

Blood clots that form in the lower part of the body may migrate to theheart and may be subsequently pumped to the lungs. Small clots can beabsorbed by the body without adverse effect. However, larger clots caninterfere with the oxygenation of blood (e.g., on the order of 3 mm indiameter and 10-30 cm in length) and can possibly cause shock or suddendeath.

Many transvenous filtering devices have been developed for installationin the vena cava to prevent especially large clots from reaching thelungs. These filters have fine wires positioned in the blood flow tocatch and hold clots for effective lysing in the blood stream. Some ofthese devices are inserted into the vena cava by dissecting the internaljugular vein in the neck or the femoral vein in the groin, inserting ametallic capsule containing a filtering device to the proper position inthe vena cava, and releasing the filtering device into the vena cava.More recently, filters have been designed for percutaneous introductioninto the vasculature.

SUMMARY

In one aspect, the invention features a filter sized and constructed tobe compressed and passed through the vasculature of a patient to beanchored against an inner wall surface of a blood vessel for capturingblood clots in a blood stream passing therethrough. The filtercomprises: an anchoring portion comprising a generally cylindricalself-expanding body formed from resilient material, the generallycylindrical body having proximal and distal ends and defining an axialdirection and having a structure of variable size diameter expandablefrom a low-profile compressed condition to a larger profile expandedcondition, wherein the resilient material urges the generallycylindrical body to radially expand and to thereby apply anchoringradial force against the inner wall surface of the blood vessel; and agenerally conical filtering portion axially aligned with the generallycylindrical body having an open proximal end coupled to the distal endof the anchoring portion and having an apical distal end, the anchoringportion and the filtering portion being substantially non-overlapping toachieve a low profile compressed condition for delivery of the filterthrough the vasculature.

Embodiments of the invention may include one or more of the followingfeatures. The generally conical filtering portion is preferably formedfrom a plurality of elongated strands arranged to form a generallyconical structure to guide blood clots in the blood stream flowingtherepast to the apical distal end of the generally conical filteringportion for lysing. The elongated strands forming the generally conicalfiltering portion are preferably constructed and arranged to maintain agenerally conical shape whether the anchoring portion is in a compressedcondition or an expanded condition. The anchoring portion and thefiltering portion are preferably constructed and arranged so that theproximal end of the filtering portion conforms to the shape of thecylindrical body of the anchoring portion. The elongated strands arepreferably fixedly attached to one another only at the apex of thegenerally conical filtering portion. The elongated strands may be formedfrom nitinol (nickel-titanium alloy), plastically deformable material,temperature-sensitive shape memory material with a transitiontemperature around body temperature, or elastic material having a coreformed from radiopaque material. The filter may be coated with a drugfor in vivo compatibility. The resilient elongated strands preferablyextend from the proximal end of the anchoring portion to the distalapical end of the filtering portion.

The elongated strands of the filtering portion may define a plurality ofneighboring filtering cells. The neighboring filtering cells arepreferably loosely coupled together at the respective areas of contactbetween neighboring cells. The neighboring cells are preferably coupledtogether by helical twisting of portions of respective elongated strandsof neighboring cells. The portion of the twisted-together elongatedstrands are preferably capable of slight mutual separation toaccommodate changes in the shapes of the cells from the expanded to thecompressed conditions. The generally conical filtering portionpreferably comprises at least two rings of cells, wherein the cells ofeach ring are of substantially equal size and are spaced substantiallythe same distance from the apical distal end of the filtering portion.The size of the cells in the rings is preferably smaller for cellscloser to the apical distal end of the filtering portion than for cellslocated a greater distance from the apical distal end of the filteringportion.

The elongated strands of the filtering portion may be spirally arrangedwith respect to one another from the proximal end of the filteringportion to the apical distal end of the filtering portion. The elongatedstrands are preferably selected to have sufficient rigidity to maintainthe generally conical shape of the filtering portion.

The self-expanding anchoring portion preferably comprises a ring ofneighboring cells. The cells of the anchoring portion are preferablyself-expanding. The cells of the anchoring portion preferably cooperateto urge the generally cylindrical body of the anchoring portion toradially expand from a compressed condition to an expanded condition.The neighboring cells of the anchoring portion are preferably fixedlycoupled together at respective areas of contact. The cells of theanchoring portion are preferably formed from one or more resilientelongated strands. When the generally cylindrical body is in acompressed condition, the cells of the anchoring portion are preferablyelongated in the axial direction.

In another general aspect, the invention features a blood clot filtercomprising: an anchoring portion formed from resilient material havingproximal and distal ends and having a generally circular transversecross-section defining an axial direction, the anchoring portion furtherhaving a structure of variable size diameter expandable from alow-profile compressed condition to a larger profile expanded condition,wherein the resilient material urges the anchoring portion to radiallyexpand and to thereby apply anchoring radial force against the innerwall surface of the blood vessel; a filtering portion axially alignedwith the generally cylindrical body having an open proximal end coupledto the distal end of the anchoring portion; and one or more hooksfixedly coupled to the anchoring portion formed from compliant materialhaving an original shape that bends under stress yet returns to itsoriginal shape when unstressed, said one or more hooks respectivelytending to project from the anchoring portion at an acute angle withrespect to the axial direction for engagement with a vessel wall, theone or more hooks further being deflectable toward the anchoring portionfor achieving a low-profile.

Embodiments of the invention may include one or more of the followingfeatures. The hooks are preferably formed from nitinol. The hookspreferably preferentially bend toward and away from the vessel wallengaging portion. The hooks are preferably formed from flat nitinol wirehaving a width dimension and having a thickness dimension substantiallysmaller than the width dimension for achieving preferential bending; theflat nitinol wire being oriented so that the thickness dimension of theflat nitinol wire coincides with a radial direction of the anchoringportion. The hooks preferably preferentially bend toward and away fromthe vessel wall engaging portion.

Among the advantages of the present invention are the following. Becausethe anchoring portion and the filtering portion have constructions thatare optimally designed for their respective functions, the filter canhave a low profile while providing a robust design that can readilyaccommodate different vessel sizes. Furthermore, the anchoring portionserves to center the filtering portion. The filtering portion of thefilter should have a small enough capture cross-section to prevent largeclots from passing therethrough. This requires a sufficient amount offiltering material (e.g., elongated strands) to reduce the capturecross-section. Since the conical filtering portion according to thepresent invention does not also have to support the filter in thevessel, smaller-sized elements can be used to form the filter to achievea lower profile. The profile of the present invention can be made small,while providing substantially the same anchoring force and substantiallythe same filtering efficiency as, e.g., a GREENFIELD® 24 Fr stainlesssteel filter (available from Medi-Tech, Inc. of Watertown, Mass.,U.S.A.). The filter designs minimally disturb blood flow, whileachieving a desirable level of filtering efficiency. Since the sizes ofthe cells of the filtering portion decrease from the proximal end to thedistal end, larger cells are positioned near the vessel walls where theflow velocity is relatively low and smaller cells are positioned in thecentral region of the vessel where the flow velocity is highest andwhere the most effective clot lysing occurs. Without being limited to aparticular theory, it is believed that clots traveling with lowervelocity do not pass through the larger size cells in the periphery ofthe conical filtering portion, but are instead guided to the apicaldistal end of the filtering portion. Clots traveling with highervelocities in the central region of the vessel, which may otherwise passthrough the larger size peripheral cells, are caught in the smaller sizecells located at the distal end of the filtering portion. Because theradial force against the vessel wall is distributed along a length ofthe vessel wall a filter according to the present invention offershigher resistance to migration as well as less trauma to the vesselwall.

Other features and advantages will become apparent from the followingdescription and from the claims. For example, the invention features aprocess for making a blood clot filter and a method for treating apatient by implanting a blood clot filter into a blood vessel of thepatient.

DESCRIPTION

FIGS. 1 and 1A are diagrammatic side and end views of a filter in anexpanded condition. FIGS. 1B and 1C are enlarged views of respectiveportions of the filter shown in FIG. 1.

FIGS. 2 and 2A are diagrammatic side and end views of a filter in acompressed condition. FIG. 2B is an enlarged view of a portion of thefilter of FIG. 2.

FIG. 3 is a plot of radial expansion force provided by a filter as afunction of the outer diameter of the filter. FIG. 3A is a diagrammaticside view of a system for measuring the radial force exerted by a filteras a function of the outer diameter of the filter.

FIGS. 4-4A are diagrammatic side views of a filter and forming mandrelsat different stages in a process for fabricating the filter shown inFIGS. 1-1b and 2-2B.

FIG. 5 is a diagrammatic side view of a filter being delivered to ablood vessel. FIG. 5A is a diagrammatic side view of a filter anchoredin a blood vessel.

FIGS. 6 and 6A are diagrammatic side and end views of a filter. FIG. 6Bis an enlarged view of a portion of the filter shown in FIG. 6.

FIGS. 7 and 7A are diagrammatic side and end views of a filter.

FIG. 8 is a diagrammatic side view of a filter. FIGS. 8A and 8B arediagrammatic end views of the filter of FIG. 8 in an expanded conditionand in a compressed condition, respectively.

Structure

Referring generally to FIGS. 1-1C and 2-2B, a blood clot filter 10includes a generally cylindrical anchoring portion 12 and a generallyconical filtering portion 14 terminating at a closed, distal apical end16. The cylindrical portion uniformly exerts an outward radial force toanchor the filter in a blood vessel (e.g., the vena cava) in which it isdisposed; the exerted force being sufficient to prevent migration of thefilter in the vessel. The generally cylindrical shape of the anchoringportion conforms to the inner wall surface of a blood vessel andproperly centers the filtering portion within the vessel. The filteringportion provides a conical meshwork across the blood vessel to catch andretain clots in the blood stream.

Cylindrical portion 12 is formed by a ring 18 of circumferentiallyarranged cells 20. Filtering portion 14 is formed by a series of threerings (22, 24, 26) of relatively loosely connected cells (28, 30, 32,respectively). The size of the cells forming the rings of the filteringportion increases from apical end 16 of the filtering portion to theproximal end 34 of the filtering portion, which is adjacent the distalend 36 of the anchoring portion.

Cells 20 of the cylindrical portion of the filter are defined byelongated strands 38 of resilient material (e.g., nitinol wire).Neighboring cells are fixedly joined together at respective regions ofcontact 40, e.g., by spot welding, as described in detail below. Fixedregions of contact 40 enable cells 20 in ring 18 to cooperate to urgethe anchoring portion into an expanded condition (FIGS. 1-1B). The fixedregions of contact 40 also prevent the elongated strands forming cells20 from rotating about each other, which might cause hinging and lockingbetween the cells in a manner distorting the cylindrical shape of theanchoring portion. In a compressed condition (FIGS. 2-2B) thelongitudinal length of cylindrical anchoring portion 12 increases.

Conical filtering portion 14 is constructed from a series of rings (22,24, 26) of relatively loosely coupled cells in a manner preserving itsgenerally conical shape, whether the filter is in a compressed conditionor an expanded condition. The filtering portion does not need to provideanchoring radial force. However, the material substance forming theconical structure has sufficient structural integrity to prevent largeclots in the blood flow from displacing the filtering structure. Thesize of the cells in the filtering portion are selected to minimallydisturb the blood flow (which would otherwise encourage occlusion of thevessel), while still achieving a desired level of blood clot filtering.

In the embodiment shown in FIGS. 1-1C and 2-2B, the cells forming thefiltering portion are coupled together by helically twisting togetherrespective portions of the elongated strands defining neighboring cells.This coupling permits some rotation about the joints in a manner thatpreserves the generally conical shape of the filtering portion, whetherthe filter is in a compressed condition or an expanded condition.

Comparing FIGS. 1B and 2B, in the expanded condition (FIG. 1B), thetwisted wire portions 52, 54, coupling neighboring cells in thefiltering portion of the filter, are tightly wrapped about each other.However, in a compressed condition (FIG. 2B), wire portions 52, 54 moveaway from (and rotate about) one another to form gaps 56. This rotationor hinging prevents the build-up of internal forces within the filteringportion, which could cause the filtering portion to bow outward into ahemispherical shape, which would result in less effective blood clotfiltering.

Referring back to FIG. 1C, a hook 44 formed from a section of flatnitinol wire is disposed within a tube 46 (e.g., a hypotube) and mountedat regions of contact 40 between neighboring cells in ring 18, whichforms the cylindrical portion of the filter. A central region of hook 38is mounted at regions of contact 40. Hook 44 is bent at its proximal anddistal ends to respectively form acute angles 48, 50 with respect to thelongitudinal axis of the cylindrical portion. The bent ends of hook 44are oriented in divergent direction to prevent migration of the filterin proximal and distal directions. The nitinol hooks easily bend toconform to the shape of the cylindrical surface of the anchoring portionto achieve a low profile for delivery of the filter. When the filter isreleased into a blood vessel, the hooks return to their bent shape forengaging an inner wall surface of the vessel. Fewer hooks may be used(e.g., three hooks symmetrically disposed about anchoring portion 12 maybe used) to achieve a lower profile for delivery of the filter.

In a presently preferred embodiment designed for filtering blood clotsin a vena cava of about 28 mm diameter, cylindrical portion 12 includessix cells formed from nitinol wire of 0.002-0.01 inch diameter, andpreferably 0.008 inch diameter (e.g., nitinol with an A_(f) between -10°C. and +5° C. and constructed so that after drawing the wire has atensile strength of about 250,000 psi to 300,000 psi, available fromShape Memory Applications of Sunnyvale, Calif., U.S.A.). Each cell inthe anchoring portion has four side portions about 13 mm in length.Filter 10 is collapsible to a diameter of 0.08 inch (about 6 Fr). Theanchoring portion has an expanded outer diameter of 30-31 mm. Thefiltering portion includes three rings of cells of decreasing size fromthe proximal end 34 to the distal apical end 16. Each of theproximalmost cells in the filtering portion has four side portions: twoproximal side portions about 13 mm in length and two distal sideportions about 15 mm in length. Each of the intermediate cells in thefiltering portion has four side portions: two proximal side portionsabout 15 mm in length and two distal side portions about 11 mm inlength. Each of the distalmost cells of the filtering portion has foursides portions: two proximal side portions about 11 mm in length and twodistal side portions about 9 mm in length. The total length of thefilter in the expanded condition is about 60 mm, with the filteringportion being about 32-34 mm in length and the anchoring portion beingabout 26-28 mm in length. Six hooks 44 are symmetrically disposed aboutthe anchoring portion at each of the fixed regions of contact 40. Hooks44 are made from flat nitinol wire about 5 mm in length, about 0.5 mm inwidth and about 0.05-0.15 mm thick.

Referring to FIG. 3, the outward radial expansion forces respectivelyexerted by six different filters of the type shown in FIGS. 1-1C and2-2B are plotted as a function of the outer diameter of cylindricalportion 12. The measured filters were designed with the specificationsrecited above. The exerted force generally varies linearly with thediameter of the anchoring portion, with the highest forces being exertedwhen the filter is in the lower profile conditions (i.e., mostcompressed). Force levels of 0.01-0.07 pounds are generally acceptablefor a typical vena cava of 12-28 mm diameter. Much higher force levelsmay cause the filter to undesirably distort the shape of the vena cava.Also, much lower force levels would not securely anchor the filter inthe vena cava and the filter may be displaced.

The number of cells in the anchoring portion and in the filteringportion may be varied to achieve larger sizes or higher forces. Forexample, to accommodate a so-called "mega-cava" having a diameter of upto 40 mm, the expanded outer diameter of the filter should be selectedto be about 42-44 mm and the number of cells in the anchoring portionshould be appropriately increased (e.g., nine cells could be used) toachieve proper outward radial force exertion to anchor the filter in thevena cava without migrating or traumatizing the vessel. Instead ofincreasing the number of cells, the thickness of the wire used to formthe cells could be suitably increased to provide the proper amount ofanchoring force. Alternatively, the exerted radial force may beincreased by providing additional welds at the distal end 36 (FIG. 1) ofthe anchoring portion at locations 126. This increases the structuralintegrity of each cell 20, providing higher spring force undercompression. The exerted radial force may alternatively be increased bychanging the wire alloy or the degree of cold work.

Referring to FIG. 3A, the outward radial force exerted by a filter wasmeasured using a force gauge 70 (e.g., a Chattillon gauge) attached toone half 72 of a solid block 74 through which cylindrical hole 76 of apreselected diameter is disposed. Block 74 was cut in half through aplane containing the longitudinal axis of cylindrical hole 76. A filterto be measured was placed in hole 76. A micrometer 80 attached to theother half 82 of block 74 was used to close the gap between the twohalves of block 74. The force exerted by the filter was measured as afunction of filter diameter by performing the measurement with a seriesof blocks with different preselected diameters.

Manufacture

Referring to FIGS. 4 and 4A, in a process for fabricating a filter 10, acylindrical thermally conductive mandrel 90 (e.g., formed from copper)is sized and constructed to conform to the desired filter size andshape. Mandrel 90 includes a plurality of anchoring pins protruding fromits outer surface in a pattern corresponding to the desired cellularpattern for the filter.

As shown in FIG. 4, the process for fabricating the anchoring portion ofthe filter includes the following steps. A wire strand 98 is bent aroundan anchoring pin 100 to form the proximal end of anchoring portion 12 ofthe filter. The two ends of wire strand 98 are pulled divergentlydownward to pins 102, 104 and through respective hypotubes 106 and 108.The strands are bent convergently further downward to pin 110 (locatedabout 23 mm distally from anchoring pin 100), below which they arehelically twisted about each other through two turns. The same steps areperformed for neighboring strands 112 and 114. Hooks 116, 118 are alsopassed through hypotubes 106, 108. The respective hypotube assembliesare joined by resistance welding under an inert gas shield using about70 ounces of force and about 10 Joules of heat.

As shown in FIGS. 4A, the process for fabricating the filtering portionincludes the following steps. The previously formed anchoring portion 12of the filter is positioned about a cylindrical portion 92 of a mandrel93 (e.g., formed from aluminum or stainless steel), which includes aconical portion 94. The ends of strand 98 are pulled divergentlydownward to pins 120, 122 (located about 22 mm proximally from thedistal end 123 of mandrel 91), below which the strands are helicallytwisted through two turns with respective ends of neighboring strands112, 114. The ends of strand 98 are convergently pulled further downwardto pin 124 (located about 8 mm proximally from the distal end 123 ofmandrel 91), below which the ends of strand 98 are helically twistedabout each other through about 4-7 turns to the apical distal end of thefiltering portion. The resulting six pairs of helically twisted strandsare passed through a short hypotube (not shown), the top of which is TIGwelded to securely fix all of the strands.

A metallic wire is wrapped about the filter/mandrel assembly to tightlysecure the relative positions of the elongated wire strands defining thecells in the anchoring and filtering portions. The filter and theforming mandrel are then placed in an oven set to a temperature of about450° C. for a period of 15 to 20 minutes. Prior to this heat treatmentthe nitinol wires are relatively malleable, but after heat treatment thenitinol wires strands preferentially maintain their shape. Once themandrel has cooled the anchoring pins are removed and the filter isremoved from the mandrel.

Referring to FIGS. 5 and 5A, a blood clot filter 10 is delivered to adesired location within a vessel 130 (e.g., a vena cava having adiameter on the order of about 20 mm) through a previously insertedteflon sheath 132. Sheath 132 having an outer diameter on the order ofabout 3 mm is inserted percutaneously, e.g., via a small opening (on theorder of 9 Fr (about 0,117 inch)) in the groin and into the femoral veinof a patient. A pusher 134, extending proximally to a location outsideof the patient, is used to advance filter 10 through the sheath. Oncethe distal end of the sheath is properly positioned in vessel 130,pusher 134 advances filter 10 to the distal end of the sheath and holdsfilter 10 in the desired position in the vessel. The sheath is thenpulled back, releasing the filter within vessel 130, as shown in FIG.5A. Once the filter is released, the sheath and the pusher can bewithdrawn from the patient as a single unit.

Referring to FIG. 5A, after the filter is released within vessel 130,the self-expanding cells of the anchoring portion urge the anchoringportion to outwardly expand against an inner wall surface 136 of vessel130 with sufficient force to prevent migration of the filter through thevessel. Within sheath 132 hooks 44 lie flat and conform to the shape ofthe cylindrical portion to allow the filter to slide through the sheath,but when the filter is released from the sheath the hooks springoutwardly from the anchoring portion of the filter for engagement withwall surface 136. The expansion of the anchoring portion imbeds hooks 44into the walls of the vessel to further secure the filter within thevessel.

We note that FIGS. 5 and 5A are not drawn to scale, but instead aredrawn diagrammatically for purposes of illustration.

In operation, the filter captures a blood clot 138 in blood flow 140(e.g., on the order of 1 liter per minute) by guiding the clot to theapical distal end 16 of the filtering portion. Captured clots 142 aremaintained in the central region of the blood flow where the velocity ishighest to achieve the most effective lysing action.

As mentioned above, the sizes of the cells in the filtering portion areselected to be small enough to capture clots of a specified size with adesired level of efficiency (e.g., with clot capturing efficiency andpatency comparable to a GREENFIELD® 24 Fr stainless steel filter,available from Medi-Tech, Inc. of Watertown, Mass., U.S.A.). Thus, it isdesirable to reduce the size of the cells to increase the efficiency ofclot capture. However, smaller cells create greater turbulence in theblood flow, encouraging clot formation on the filter that may result inthe occlusion of a vessel. A filter according to the invention minimallydisturbs blood flow, while achieving a desirable level of filteringefficiency. The sizes of the cells in the filtering portion decrease thecloser they are to the apical distal end 16. Thus, cell size in thefiltering portion varies inversely with blood flow velocity: largercells are positioned near the vessel walls where the flow velocity isrelatively low and smaller cells are positioned in the central region ofthe vessel where the flow velocity is highest. Clots traveling withlower velocity do not pass through the larger size cells in theperiphery of the conical filtering portion, but are instead guided tothe apical distal end of the filtering portion. Clots traveling withhigher velocities in the central region of the vessel, which mayotherwise pass through the larger size peripheral cells, are caught inthe smaller size cells located at the distal end of the filteringportion.

Other Embodiments

Referring to FIGS. 6-6B, a blood clot filter 150 includes a generallycylindrical anchoring portion 152 and a generally conical filteringportion 154. Anchoring portion 152 includes a ring of cells 156 and isconstructed in a similar manner as anchoring portion 12 of filter 10,shown in FIGS. 1-1C and 2-2B. Filtering portion 154 is formed from sixspirally arranged legs 158 terminating at an apical distal end 160.

Legs 158 of the filtering portion of the filter are twisted through 90°over a length of about 32-34 mm. Twisting legs 158 creates a series ofspirally arranged cells 162. The projection of legs 158 in a planetransverse to the longitudinal axis of the anchoring portion revealsthat the cells defined by legs 158 decrease in size from the peripheraledge of the filtering portion to the apical center; the amount ofreduction being determined by the twist pitch (degrees of rotation perunit length) and the number of legs 158 in the filtering portion. Thisreduction in cell size achieves an advantage similar to the advantageachieved by the reduction in cell size in the embodiment of FIGS. 1-1Cand 2-2B, as described above.

As shown in FIG. 6B, legs 158 are formed from pairs of elongated strandsof resilient material (e.g., nitinol wire) 164, 166 helically twistedabout each other. Strands 164, 166 correspond to the respective ends ofstrands 168 that are bent into a V-shape to form the proximal end ofanchoring portion 152. Twisting strands 164, 166 increases the rigidityof legs 158 for maintaining the structural integrity of the generallyconical filtering portion. Increasing the rigidity of legs 158 alsoprevents clots from forcing their way past the filter by displacing therelative positions of the legs.

Referring to FIGS. 7-7A, in another filter embodiment 170, a generallycylindrical anchoring portion 172 is constructed in a similar manner asanchoring portion 12 of filter 10, shown in FIGS. 1-1C and 2-2B. Agenerally conical filtering portion 174 is formed from six spirallyarranged legs 176 terminating at an apical distal end 178.

Legs 176 of filtering portion 174 are twisted through 90° over a lengthof about 32-34 mm, as in the filter embodiment shown in FIGS. 6-6B,creating a ring of spirally arranged cells 180. However, each leg 176 isformed from the continuation of a single elongated strand (formed from,e.g., nitinol wire) from the anchoring portion. To increase thestructural integrity of the anchoring portion and the filtering portion,a series of spot welds 182 are provided at the distal end of theanchoring portion, joining strands 184, 186 that define cell 188.

As shown in FIGS. 8-8B, the anchoring portion 190 of a filter 192 may beformed from flat strands 200 (e.g., formed from superelastic materialsuch as nitinol wire) having a rectangular cross-section. The anchoringportion of the filter is shown in an expanded condition in FIG. 8 and ina compressed condition in FIG. 8A. The flat strands are arranged in theform of a ring of cells 202 (e.g., six cells), with the number and sizeof the cells being selected to provide a desired level of anchoringforce. The width dimension 204 (on the order of 0.5-0.7 mm wide) of flatstrands 200 is oriented radially and the thickness dimension 206 (on theorder of 0.05-0.15 mm thick) is oriented circumferentially. This strandorientation provides a high radial force-to-compressed profile ratio.Also, use of flat strands facilitates manufacture of the filter becausethere is more strand material available for welding. A filtering portion194 (e.g., a conical filtering portion) may be formed from spirallyarranged wires as shown or may be formed from rings of cells, as in thefilter of FIG. 1. The filtering portion may be formed from the extensionof flat strands 200. Alternatively, a filtering portion may be formedfrom round wire that may be joined to the flat strand anchoring portionby welding with a hypotube arranged as a universal-type hinge, or byusing an adhesive or sutures.

Although the invention has been described in connection with blood clotfiltering in the vena cava, the present invention would also be usefulfor filtering clots in other areas of the vascular anatomy. For example,blood clot filtering may be useful in vessels leading to the brain. Thefilter used in such applications would be constructed of appropriatesize and of appropriate material to provide proper anchoring forceagainst an inner wall surface of the vessel in which the filter isdisposed.

In further embodiments, the respective strands 38 and hooks 44 inregions of contact 40 (FIG. 1) in the anchoring portion of the filtermay be joined together using laser welding along a length of about,e.g., 2-3 mm, instead of using a hypotube and resistance welding.

In other embodiments, the filter may be of the non-self-expanding type,preferably delivered using a catheter having an expandable balloon. Thecells can be made of plastically deformable material, which may be, forexample, tantalum, titanium, or stainless steel.

In still other embodiments, the filter may be formed of atemperature-sensitive shape memory material with a transitiontemperature around body temperature. The filter may then be delivered ina compressed condition in one crystalline state and expanded bycrystalline phase transformation when exposed to body temperature.

In other embodiments, at least a portion of the filter may be formedfrom nitinol wire having a core of tantalum wire or other radiopaquematerial, as described in U.S. Ser. No. 07/861,253, filed Mar. 31, 1992and U.S. Ser. No. 07/910,631, filed Jul. 8, 1992, both of which areherein incorporated by reference. This enhances the radiopacity of thefilter so that the filter may be viewed using X-ray fluoroscopy tomonitor placement and operation of the filter.

In still other embodiments, the filter may be coated with a drug for invivo compatibility prior to delivery into the body. For example, thefilter may be coated with heparin, as described in U.S. Pat. Nos.5,135,516 and 5,304,121, which are herein incorporated by reference.

Other embodiments are within the scope of the claims.

What is claimed is:
 1. A filter sized and constructed to be compressedand passed through the vasculature of a patient to be anchored againstan inner wall surface of a blood vessel for capturing blood clots in ablood stream passing therethrough, said filter comprisingan anchoringportion comprising a generally cylindrical self-expanding body formedfrom resilient material, said generally cylindrical body having proximaland distal ends and defining an axial direction and having a structureof variable size diameter expandable from a low-profile compressedcondition to a larger profile expanded condition, wherein said resilientmaterial urges said generally cylindrical body to radially expand and tothereby apply anchoring radial force against the inner wall surface ofthe blood vessel, and a generally conical filtering portion, axiallyaligned with said generally cylindrical body having an open proximal endcoupled near the distal end of said anchoring portion and having anapical distal end.
 2. The filter of claim 1 wherein said anchoringportion and said filtering portion are substantially non-overlapping toachieve a low profile compressed condition for delivery of the filterthrough the vasculature.
 3. The filter of claim 1 wherein said generallyconical filtering portion is formed from a plurality of elongatedstrands arranged to form a generally conical structure to guide bloodclots in the blood stream flowing therepast to the apical distal end ofsaid generally conical filtering portion for lysing.
 4. The filter ofclaim 3 wherein said elongated strands forming the generally conicalfiltering portion are constructed and arranged to maintain a generallyconical shape whether said anchoring portion is in a compressedcondition or an expanded condition.
 5. The filter of claim 3 whereinsaid elongated strands are fixedly attached to one another only at theapex of said generally conical filtering portion.
 6. The filter of claim3 wherein said elongated strands define a plurality of neighboringfiltering cells.
 7. The filter of claim 6 wherein neighboring filteringcells are loosely coupled together at the respective areas of contactbetween neighboring cells.
 8. The filter of claim 7 wherein neighboringcells are coupled together by helical twisting of portions of respectiveelongated strands of neighboring cells.
 9. The filter of claim 8 whereinthe portion of the twisted together elongated strands are capable ofslight mutual separation or rotation to accommodate changes in theshapes of said cells from the expanded to the compressed conditions. 10.The filter of claim 6 wherein said generally conical filtering portioncomprises at least two rings of cells, wherein the cells of each ringare of substantially equal size and are spaced substantially the samedistance from the apical distal end of said filtering portion.
 11. Thefilter of claim 10 wherein the size of the cells in the rings is smallerfor cells closer to the apical distal end of said filtering portion thanfor cells located a greater distance from the apical distal end of saidfiltering portion.
 12. The filter of claim 3 wherein said elongatedstrands are spirally arranged with respect to one another from theproximal end of said filtering portion to the apical distal end of saidfiltering portion.
 13. The filter of claim 12 wherein said elongatedstrands are selected to have sufficient rigidity to maintain thegenerally conical shape of said filtering portion in a flowing bloodstream.
 14. The filter of claim 3 wherein said elongated strands areformed from nitinol.
 15. The filter of claim 3 wherein said elongatedstrands are formed from plastically deformable material.
 16. The filterof claim 3 wherein said elongated strands are formed fromtemperature-sensitive shape memory material with a transitiontemperature around body temperature.
 17. The filter of claim 3 whereinsaid elongated strands are formed from elastic material having a coreformed from radiopaque material.
 18. The filter of claim 1 wherein saidanchoring portion and said filtering portion are constructed andarranged so that the proximal end of said filtering portion conforms tothe shape of the cylindrical body of said anchoring portion.
 19. Thefilter of claim 1 wherein said filter is coated with a drug for in vivocompatibility.
 20. The filter of claim 1 wherein said self-expandingbody of said anchoring portion comprises a ring of neighboring cells.21. The filter of claim 20 wherein the cells of said anchoring portionare self-expanding.
 22. The filter of claim 20 wherein the cells of saidanchoring portion cooperate to urge said generally cylindrical body ofsaid anchoring portion to radially expand from a compressed condition toan expanded condition.
 23. The filter of claim 20 wherein neighboringcells are fixedly coupled together at respective areas of contact. 24.The filter of claim 20 wherein the cells of said anchoring portion areformed from one or more resilient elongated strands.
 25. The filter ofclaim 20 wherein when said generally cylindrical body is in a compressedcondition said cells are elongated in the axial direction.
 26. Thefilter of claim 1 wherein said anchoring portion and said filteringportion are formed from resilient elongated strands extending from theproximal end of said anchoring portion to the distal apical end of saidfiltering portion.
 27. The filter of claim 1 further comprising one ormore hooks mounted on said anchoring portion.
 28. A filter sized andconstructed to be compressed and passed through the vasculature of apatient for anchoring against an inner wall surface of a blood vesselfor capturing blood clots in a blood stream passing therethrough, saidfilter comprisingcylindrical anchoring means having proximal and distalends and defining an axial direction and having a structure of variablesize diameter expandable from a low-profile compressed condition to alarger profile expanded condition, and conical filtering means axiallyaligned with said generally cylindrical body, the filtering means havingan open proximal end coupled to the distal end of said anchoring portionand having an apical distal end, wherein said cylindrical anchoringmeans and said conical filtering means being substantiallynon-overlapping to achieve a low profile compressed condition fordelivery of the filter through the vasculature.
 29. A filter sized andconstructed to be compressed and passed through the vasculature of apatient to be anchored against an inner wall surface of a blood vesselfor capturing blood clots in a blood stream passing therethrough, saidfilter comprisingan anchoring portion comprising a generally cylindricalbody formed from resilient material having proximal and distal ends andhaving a generally circular transverse cross-section defining an axialdirection, said anchoring portion further having a structure of variablesize diameter expandable from a low-profile compressed condition to alarger profile expanded condition, wherein said resilient material urgessaid anchoring portion to radially expand and to thereby apply anchoringradial force against the inner wall surface of the blood vessel, afiltering portion axially aligned with said generally cylindrical bodyhaving an open proximal end coupled to the distal end of said anchoringportion, and one or more hooks fixedly coupled to said anchoring portionformed from compliant material having an original shape that bends understress yet returns to its original shape when unstressed, said one ormore hooks respectively tending to project from said anchoring portionat an acute angle with respect to the axial direction for engagementwith a vessel wall, said one or more hooks further being deflectabletoward said anchoring portion for achieving a low-profile.
 30. Thefilter of claim 29 wherein said hooks are formed from nitinol.
 31. Thefilter of claim 29 wherein said hooks preferentially bend toward andaway from said vessel wall engaging portion.
 32. The filter of claim 31wherein said hooks are formed from flat nitinol wire having a widthdimension and having a thickness dimension substantially smaller thansaid width dimension for achieving preferential bending, said flatnitinol wire being oriented so that the thickness dimension of said flatnitinol wire coincides with a radial direction of the anchoring portion.